1. Field of the Invention
This invention relates generally to magnetic disk drive technology, and more specifically to a suspension for use in a data storage disk drive.
2. Description of the Background Art
Disk drives are used for nonvolatile memory storage in computer systems. Disk drives have at least one magnetic recording head mounted on a slider. An actuator positions the slider over a magnetic disk for writing and reading information on the disk. The mechanism which connects the slider to the actuator is called a suspension. Conventional suspensions have several shortcomings including poor damping characteristics and other characteristics which contribute to increased track misregistration (TMR) as discussed in detail below. The present invention is a suspension which has superior damping characteristics and optimized TMR performance.
Some of the features of conventional suspensions are illustrated in FIGS. 1a and b. Most suspensions have a mounting plate 102 having a hole 104 with a raised lip (not shown) for swaging into a rigid arm. The suspension has a hinged portion 106 and a rigid load beam portion 108. The hinge portion 106 imparts a spring action to the load beam 108 which forces the slider toward the disk. Each load beam has an associated head gimbal pivot point. There are at least two head gimbal structures used in suspensions which differ in the slider attachment to the suspension. The location of the head gimbal pivot point depends on which of these structures is used. For the structure illustrated in FIGS. 1a and b, the slider 110 is attached to a flexure 112 forming a head gimbal which is also called a slider gimbal. In this structure, there is a dimple (not shown) usually formed on the load beam which serves as the pivot point for the head gimbal. The dimple may also be formed on a flexure member which is attached directly to the load beam. The location of the dimple establishes the head gimbal pivot point relative to the load beam. Another type of suspension does not use a dimple to define the head gimbal pivot point and is known as a dimpleless design. This design is taught in U.S. Pat. No. 5,198,945 and U.S. Pat. No. 5,912,788. Referring to FIG. 2, the pivot point for the head gimbal is the intersection of a torsional axis 32 of the flexure and a pitch axis 30. FIG. 3 shows how the head gimbal is constructed to include a load beam.
Referring to FIGS. 1a and 1b, a suspension generally has a torsional vibration mode which rotates about a torsional axis 114. To provide stiffness to the load beam 108 a portion of each of the outside edges 118 is bent out of the plane of the load beam to form a flange 116 as shown in FIG. 1a. Most commonly the flanges are bent away from the disk, but they can also be bent toward the disk. The flat portion 120 of the load beam between the flanges 116 defines the plane of the load beam. In conventional suspensions the load beam 108 and the hinge portion 106 are formed from the same continuous sheet of material. The forming process for hinge portion 106, which produces spring action, alters the relative position of the load beam portion 108 to the mounting plate 102. Two parameters of this relative position are sag and formed area flatness (FAF), which are described in detail below.
An important index of the performance of a disk drive is track misregistration (TMR). Track misregistration is a measure of the distance from the recording head to the center of the desired track on the disk and represents an undesired misalignment of the head with respect to the center of the track. As the offset of the torsional axis of the suspension increases relative to the pivot point of the slider gimbal the TMR also increases. This is because the offset of the torsional axis to the pivot point of the slider acts as a lever for lateral slider motion. This lateral motion contributes directly to TMR.
As illustrated in FIGS. 4a, c, and e (in which the vertical scale is exaggerated), sag is shown as the location or offset of the flat portion of the load beam 403 with respect to the mounting plate 401. The amount of sag in a conventional suspension determines the degree to which the torsional axis is misaligned with the pivot point of the slider gimbal. FIG. 4a shows a side view of a suspension with positive sag 410. FIG. 4a includes a view of the mounting plate 401, the hinge portion 402, the load beam portion 403, and the slider 404. A positive sag exists when the gimbal pivot point of a suspension is closer to the disk than the axis of torsional rotation. FIG. 4b shows an end view of the load beam 406, the slider 404 and the pivot point 407 for the case of positive sag. Because of positive sag, the torsional axis 405 is above the slider gimbal pivot point 407 which results in additional movement of the recording head 408 and increased TMR 409. FIG. 4c shows a case of optimal sag 410 wherein the torsional axis intersects the gimbal pivot point. This results in minimal TMR 409 and good disk drive performance. In FIG. 4c, the mounting plate 401, hinge portion 402, load beam 403, and slider 404 are similar as in FIG. 4a. In FIG. 4d it is shown that at optimal sag the torsional axis 405 intersects the pivot point 407. FIG. 4e illustrates negative sag 410. FIG. 4f illustrates that for negative sag the torsional axis 405 is below the pivot point 407 resulting in greater movement of the recording head 408 and increased TMR 409.
The distance between the torsional axis and the pivot point acts as a lever arm. Since the slider is constrained by the presence of the disk from rotating as it flies on the disk surface, this lever transfers the torsional motion of the load beam as linear motion to the slider. This linear motion is perpendicular to the direction of the recorded track. This sideway linear motion of the slider relates directly to TMR because the recording head attached to the slider moves away from the center of the track. Therefore, from FIG. 4a, b, c, d, e, and f, it can be seen that the farther the torsional axis is away from the pivot point the greater the effect on TMR. Therefore, it is desirable to keep the distance between the torsional axis and the pivot point which acts as a lever arm as small as possible to minimize TMR.
Another parameter of the suspension which must be controlled is the Formed Area Flatness (FAF). Referring to FIGS. 1a and 1b, FAF is defined as the coplanarity of the flat portion 120 of the load beam with respect to the mounting plate 102. In other words, FAF is a measurement of the angular tilt of the flat portion 120 of the load beam using the mounting plate as a reference surface. FAF and sag are interdependent in conventional suspensions which have the same continuous material used for forming both the hinge and the load beam. Generally a conventional suspension with optimized FAF will have sub optimum sag and conversely a suspension with optimized sag will have sub optimal FAF. FAF primarily controls how much power, or gain, is in the torsional vibrations of the suspension whereas sag primarily controls the effect of torsional vibrations on TMR. The effects of both FAF and sag are important factors in the dynamic performance of the suspension.
What is needed is a suspension in which the sag can be controlled independently from FAF and the hinge forming process. For the torsional vibrations that do occur, it is desired to have the gain of those vibrations substantially reduced by the damping characteristics of the suspension.